eclipse plugin: Don't log missing files when the path is null or empty

utils/umc_dev/org.wesnoth/src/org/wesnoth/preferences/Preferences.java

@@ -131,7 +131,6 @@ public class Preferences extends AbstractPreferenceInitializer
         getPreferences( ).setValue( Constants.P_INST_DEFAULT_INSTALL, newInstallName );
     }
 
-
 	/**
 	 * Returns a new Paths object based on the specified install name
 	 * @param installName The install name used for the paths

utils/umc_dev/org.wesnoth/src/org/wesnoth/utils/ResourceUtils.java

@@ -319,7 +319,7 @@ public class ResourceUtils
 	public static boolean isValidFilePath( String filePath )
 	{
 	    boolean valid = filePath != null && !filePath.isEmpty() && new File(filePath).exists();
-	    if ( valid == false )
+	    if ( valid == false && ! StringUtils.isNullOrEmpty( filePath ) )
 	        Logger.getInstance().logWarn("The file does not exist or is null: " + filePath); //$NON-NLS-1$
 	    return valid;
 	}